N/E fault RCD not tripping ??

[Buzz1980]

Hi all

can anyone give me the logic why the RCD wouldn’t trip on a neutral/earth fault. Testing a extension where they have added in 5 additional sockets to the upstairs ring had been powered up and nothing tripping with household appliances plugged in and working. Carrying out IR tests to find had dead short on one leg of the neutral and earth between 2 sockets but there was no issue of tripping ? RCD checks out ok with test button and RCD tests. Have disconnected this leg and then fitted fuse spur off the ring to carry the additional sockets now as to late to add in additional cable, so extension spurred off ring using fuse spur. Would it be the case that it had seized a bit RCD or that its connected to a TNCS system?? Replies much appreciated ?

 

[telectrix]

it has been known for a N-E fault to inhibit the RCD from tripping, even on the test button. don't know the theory behind it, but i have seen it a couple of times.

 

[westward10]

It may not be creating an imbalance. Just because the neutral and cpc have become connected does not necessarily mean enough will be diverted away from the neutral for the imbalance.

 

[Buzz1980]

Aww ok thanks for the reply ?. Had also a couple of other issues never seen before.

Trying to carry out Zs test on upstairs lights and found that it would carry out test as error code of >65 fre. Looked at the manuals and states supply frequency to high for test. So I disconnected the dimmer switch and managed to carry out test. Seems the dimmer is causing the frequency issue but don’t know why this would be as should be rated for 50hz ?

another issue think I know the issue on this one though IR test on socket circuit live to earth dead short. Join live and neutral together test down to earth >999mega ohms. Thinking with me going live to earth IR test travelling though USB ports on the sockets ? I noticed behind a cabinet was causing reading to be dead short. This is the only reason I can think of as unable to switch off USB ports ?

 

[James]

If you know there are usb sockets in the circuit, testing N-E insulation is a bad idea.
you can't say there is a dead short if testing with an insulation resistance tester.

I would only consider something to be a dead short if measured below 1 ohm.

 

[Buzz1980]

Hi yeah I tested live to earth but coming up 0.00mega ohm but link neutral and live together then test then all clear >999 mega ohm so it’s like I’m testing through the USB when doing live to earth on the IR tests ?

 

[Buzz1980]

I’m thinking I would have to disconnect sockets and connector block to carry out live to earth then neutral to earth ?

 

[Grant1987]

I’m thinking I would have to disconnect sockets and connector block to carry out live to earth then neutral to earth ?

You should be doing a Bridge test or disconnecting sensitive equipment or putting it down as limited on test certificate.

 

[westward10]

Hi yeah I tested live to earth but coming up 0.00mega ohm but link neutral and live together then test then all clear >999 mega ohm so it’s like I’m testing through the USB when doing live to earth on the IR tests ?

If your neutral is disconnected from the neutral bar you should not get zero as your linked neutral/line to earth was clear.

 

[Grant1987]

If your neutral is disconnected from the neutral bar you should not get zero as your linked neutral/line to earth was clear.

Good point ?

 

[Wilko]

Sorry I may not have followed things here fully, but if you are reading zero N - E but everything works etc then I’m thinking perhaps you’re recording the excellent N - E connection at the cutout?

 

[Buzz1980]

I don’t think so as I managed to trace the fault and link between two sockets N to E short. But that may be a case in future works.?

 

[Buzz1980]

But regarding the USB sockets. If I have not disconnected the neutrals from neutral bar and tested between live and earth and I am getting low reading basically zero near enough, this should be because the USB points are bringing the reading down as I will be going through the earth and being TNCS will be linked to neutral so will be testing the USB point. As when I link out live and neutrals at the consumer unit and go to a socket then test live to earth insulation resistance it comes out clear. >999mega ohms. Now if that was a damaged cable that would be picked up, does that make sense ?

 

[Grant1987]

But regarding the USB sockets. If I have not disconnected the neutrals from neutral bar and tested between live and earth and I am getting low reading basically zero near enough, this should be because the USB points are bringing the reading down as I will be going through the earth and being TNCS will be linked to neutral so will be testing the USB point. As when I link out live and neutrals at the consumer unit and go to a socket then test live to earth insulation resistance it comes out clear. >999mega ohms. Now if that was a damaged cable that would be picked up, does that make sense ?

What voltage are you doing insulation resistance test?

 

[Buzz1980]

250volts to see if ok then 500volts after that test ?

 

[Grant1987]

250volts to see if ok then 500volts after that test ?

Ok well where did you get the >999Mohms?

 

[Wilko]

I don’t think so as I managed to trace the fault and link between two sockets N to E short. But that may be a case in future works.?

Oops, sorry I missed that

 

[Buzz1980]

On both tests at 250v and 500v. I tested live to earth at 250volt seen that it was very low near enough zero, now this has been connected to a RCD and MCB for years this circuit. So I then joined live and neutrals together at consumer unit then tested live to earth IR test at nearest socket comes up >999mega ohms. So I am thinking I’m testing through the USB points when carrying out live to earth only with neutrals still connected to N bar ?? That make sense

 

[Grant1987]

Ok well where did you get the >999Mohms?

Googling it now are you? Lol

 

[Buzz1980]

Sorry googling what ???

 

[Grant1987]

On both tests at 250v and 500v. I tested live to earth at 250volt seen that it was very low near enough zero, now this has been connected to a RCD and MCB for years this circuit. So I then joined live and neutrals together at consumer unit then tested live to earth IR test at nearest socket comes up >999mega ohms. So I am thinking I’m testing through the USB points when carrying out live to earth only with neutrals still connected to N bar ?? That make sense

Testing at 250v is only beneficial when there is a fault so that you don’t need to subject circuit to 500v however you need to carry out insulation resistance test at 500v to record accurate fault reading and true scale of insulation. I’m trying to test you here ?

 

[Grant1987]

Testing at 250v is only beneficial when there is a fault so that you don’t need to subject circuit to 500v however you need to carry out insulation resistance test at 500v to record accurate fault reading and true scale of insulation. I’m trying to test you here ?

I’m here to learn just like yourself and Aslong as it’s done appropriately then there is no harm. Best way is to ensure everything is unplugged from sockets

 

[Buzz1980]

Yeah appreciate the response, but it has just got me thinking why it will show up dead short L- E but then join neutral and live together, IR test at nearest socket and comes up clear ?

 

[Grant1987]

Yeah appreciate the response, but it has just got me thinking why it will show up dead short L- E but then join neutral and live together, IR test at nearest socket and comes up clear ?

Doesn’t sound right to me butty, needs further investigation without a doubt

 

[Grant1987]

Doesn’t sound right to me butty, needs further investigation without a doubt

If it’s dead short L-E should be a banger? Is it old consumer?

 

[Grant1987]

If it’s dead short L-E should be a banger? Is it old consumer?

These are the jobs I hate, because you have to explain to customer that it’s not right even though it appears fine and well. Don’t leave it there find the fault and charge accordingly you’re the last sparky there so when you close that door it’s your neck on the block

 

[Buzz1980]

No it’s 6 months old, that’s why I am thinking I’m testing through the USB ports only thing I can think of as if it was a dead short then it would show up on L- N together and test to earth as well but it’s clear ?

 

[Grant1987]

No it’s 6 months old, that’s why I am thinking I’m testing through the USB ports only thing I can think of as if it was a dead short then it would show up on L- N together and test to earth as well but it’s clear ?

Well disconnect the usb sockets put the cables in connector blocks then test. Make sure you are disconnecting the circuit in question out of bar/mcb to ensure it’s definitive. I’m a bit confused here sorry it’s late and been working 14 hours again ?‍ You’ve been called back after 6 months and it’s tripping?

 

[Grant1987]

Well disconnect the usb sockets put the cables in connector blocks then test. Make sure you are disconnecting the circuit in question out of bar/mcb to ensure it’s definitive. I’m a bit confused here sorry it’s late and been working 14 hours again ?‍ You’ve been called back after 6 months and it’s tripping?

The USB ports will 100% interfere with you’re readings and give false readings disconnect them. So the fault is on the sockets? Unplug everything and then carry out insulation resistance. Once established that the fault is on the ring main. Most likely pain in the arse is N-E fault split the legs at consumer go to half way point of ring (take an educated guess) then you will know which side the fault is on)

 

[Buzz1980]

Sorry no the consumer unit is 6months old, this was a extension that got done and they require test cert. yeah think best to disconnect but then test kind of pain in the arse tbh is it disconnecting circuits ?

 

[Grant1987]

Sorry no the consumer unit is 6months old, this was a extension that got done and they require test cert. yeah think best to disconnect but then test kind of pain in the arse tbh is it disconnecting circuits ?

Ah I wish I was local to you now I would help you out. Are you at the property now?

 

[Buzz1980]

But I have IR test with line and neutral connected and down to earth all clear only coming dead short when I do live to earth, which if it was damaged cable then it would show up on line/neutral together to earth ??

 

[Buzz1980]

Must be USB port that brings it down, I’m thinking when IR testing down the earth it’s coming back on the neutral and causing it to short through the USB ?? ?

 

[Grant1987]

But I have IR test with line and neutral connected and down to earth all clear only coming dead short when I do live to earth, which if it was damaged cable then it would show up on line/neutral together to earth ??

Right forget about joining them, do them individually I’m certain your lays between N-E put your 2 legs into a connector block for earth and then the other 2 legs for neutral in a connector block. Test them with your probes. You will probs get a big fat 0. Are you still at the property if you are I will talk you through it

 

[Buzz1980]

Sorry not there anymore, left 4 hours ago ?

 

[Grant1987]

Right forget about joining them, do them individually I’m certain your lays between N-E put your 2 legs into a connector block for earth and then the other 2 legs for neutral in a connector block. Test them with your probes. You will probs get a big fat 0. Are you still at the property if you are I will talk you through it

Fault finding is

Sorry not there anymore, left 4 hours ago ?

Ah gutting hopefully they got power?

 

[Buzz1980]

Yeah it’s not tripping hasn’t tripped in years ?

 

[Grant1987]

Fault finding is

Ah gutting hopefully they got power?

If you’re struggling I’ll message you direct with my phone number and you can ring me direct tomorrow ?

 

[Grant1987]

Yeah it’s not tripping hasn’t tripped in years ?

With a dead short? ?

 

[Grant1987]

With a dead short? ?

Ok I’ll leave you to it lol, doesn’t sound good to me

 

[Buzz1980]

I know that’s why I don’t think it’s a dead short and the reading is from the USB points, live to earth IR test, goes down the earth which is also connected to the neutral at the supply TNCS will go through this neutral back around all socket and through the line conductor hence giving me a short circuit, but when line and neutral connected together all clear ??

 

[Ziggie]

If you know there are usb sockets in the circuit, testing N-E insulation is a bad idea.
you can't say there is a dead short if testing with an insulation resistance tester.

I would only consider something to be a dead short if measured below 1 ohm.

But regarding the USB sockets. If I have not disconnected the neutrals from neutral bar and tested between live and earth and I am getting low reading basically zero near enough, this should be because the USB points are bringing the reading down as I will be going through the earth and being TNCS will be linked to neutral so will be testing the USB point. As when I link out live and neutrals at the consumer unit and go to a socket then test live to earth insulation resistance it comes out clear. >999mega ohms. Now if that was a damaged cable that would be picked up, does that make sense ?

The USB installation blurb differs between brands. I fitted a LAP usb socket and it states that you can do installation test at 500 V but others state that they must be disconnected.

first thing mentioned on my insulation test is that dimmers should be disconnected for the test.

Short circuit between live conductors after the RCD will not ensure it’s tripping ( RCD guide Eaton.EU) may apply and example shown is N to E.

 

[richy3333]

This might be of use to you:

https://uk.megger.com/electrical-te...on/june-2015/testing-usb-wall-mounted-sockets

It should also be noted that the electronic circuitry inside the SMPS could easily be damaged by a 500 V insulation test injected across it. Consequently the USB port may fail to function following a 500 V Line to Neutral insulation test